This invention relates to hydraulic servos.
A typical servo senses movement to control fluid flow through a valve which regulates hydraulic pressure on one side of the piston which is connected to the valve by a spring. The piston moves in response to the error signal produced by the sensed movement and causes the valve to close to a null position at a piston equilibrium position.
In an ideal, completely frictionless servo system, as soon as the valve opens slightly, the piston begins to move to reestablish the valve in the null position. But since actual systems are not frictionless, it is necessary for the valve to open enough for the piston to develop force to overcome friction. The valve and the apparatus interconnecting it with the sensed movement should have a positive rate, for goof servo stability. But to establish some additional valve opening in response to sensed movement to overcome friction, a negative rate (overshoot) must be introduced, although the overall rate must still be positive.
An example of a technique for meeting these objectives is taught in my U.S. Pat. No. 3,757,639, titled NEGATIVE HYDRAULIC RATE DEVICE, which issued on Sept. 11, 1973 and is commonly owned herewith. There the negative rate is established through the use of a bellows which senses the hydraulic pressure established by a positive rate flapper valve. The bellows oppose valve movement with an opposing force that decreases with increasing valve flow, thus providing a negative rate to overcome friction.